Ferritic stainless steel for egr system

ABSTRACT

Disclosed is a ferritic stainless steel for an EGR system, more particularly, a ferritic stainless steel for an EGR system which may suppress the ferritic stainless steel from being discolored and which improves the moldability, oxidation resistance, and corrosion resistance by including iron (Fe) as a base material, about 18 to 20% by weight of chromium (Cr) based on a total weight of an alloy, molybdenum (Mo), carbon (C) and niobium (Nb).

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. §119 to Korean PatentApplication No. 10-2012-0158666, filed on Dec. 31, 2012, in the KoreanIntellectual Property Office, the disclosure of which is incorporatedherein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a ferritic stainless steel for anexhaust gas recirculation (EGR) system, particularly a ferriticstainless steel for an EGR system, which resists a color change duringthe brazing thereof. More particularly, the present invention provides aferritic stainless steel that resists color change by controlling thecomposition ratio of the ferritic stainless steel, and that furtherimproves the moldability, oxidation resistance, and corrosionresistance.

2. Description of the Related Art

While air pollution caused by factories, heating, power generationfacilities, and the like is decreasing, air pollution caused by exhaustgas from automobiles is increasing. Exhaust gas from automobiles isexhausted at high concentrations in densely populated areas and housingareas near roads, thereby causing great harm to people's health.Accordingly, a significant amount research and development has beendirected towards treating exhaust gas from automobiles.

In general, an automobile is driven by an internal combustion engine,and the engine is operated by the combustion of fossil fuel. During thecombustion of fossil fuel, hazardous materials that cause environmentalpollution are discharged. Such hazardous materials include, for example,carbon monoxide (CO), nitrous oxide (NOx), carbon dioxide (CO₂), sulfuroxide (Sox) and the like.

In particular, the nitrous oxide (NOx) included in the exhaust gas isresponsible for acid rain and smog. Nitrous oxide (NOx) furtherirritates the eyes and the respiratory organs, thereby causing symptomssuch as the release of saliva, sore throat, dizziness, headache, andvomiting, and further leads to plant death. Accordingly, nitrous oxide(NOx) is regulated as a main air pollutant, and numerous devices forreducing the discharge of nitrous oxide (NOx) have been developed.

Among these devices, an EGR system-related technology has been activelydeveloped. The EGR system consists of an EGR cooler, an EGR pipe, an EGRvalve, and the like. When a fuel-air mixture (i.e., a gas in which fueland air are mixed) is combusted in a cylinder during the explosionstroke of an engine, the combustion temperature may be decreased byrecirculating a part of the exhaust gas into an intake system of theengine. This suppresses nitrous oxide (NOx) from being produced by thereaction of nitrogen (N₂) and oxygen (O₂) in the combustion air at hightemperature. That is, the EGR system is a system that decreases theamount of nitrous oxide (NOx) produced by lowering the combustiontemperature when the fuel-air mixture is combusted after a part of theexhaust gas is returned to the intake system.

Austenitic stainless steel is one material that has been commonly usedfor the constituent parts of the EGR system. However, nickel (Ni), whichis expensive, is included therein in a large amount. Thus, the use ofaustenitic stainless steel is problematic due to the steady increase incost caused by the high cost of nickel and the price instability ofnickel. For popularization of the EGR system, it is essential to reducecosts associated therewith, and it is necessary to use a reasonablypriced and appropriate materials.

In order to reduce the cost as described above, there has been anattempt to replace a portion to which austenitic stainless steel isapplied with ferritic stainless steel, which is relatively inexpensive.However, the EGR system is subjected to a brazing process of melting ametal insert to be bonded to a base metal in a batch furnace orcontinuous furnace. This disadvantageous because titanium (Ti) which isincluded in a conventional ferritic stainless steel reacts with nitrogen(N₂) during the brazing thereof to produce titanium nitride (Ti—N) onthe surface thereof. This results in a change in the color of thematerial.

Further, the conventional ferritic stainless steel has insufficientmoldability, and thus is disadvantageous in that an orange peelphenomenon is generated (i.e., modified lines or wrinkles are generatedon a surface that does not include cracks).

In addition, the conventional ferritic stainless steel isdisadvantageous in that the ferritic stainless steel has lower oxidationresistance and corrosion resistance than austenitic stainless steel.

Thus, what is needed is a ferritic stainless steel for an ERG system,which is inexpensive, has improved moldability, oxidation resistance andcorrosion resistance, and which further avoids discoloration thereofduring brazing.

SUMMARY OF THE INVENTION

The present invention provides a ferritic stainless steel for an EGRsystem, which is inexpensive and has improved moldability, oxidationresistance, and corrosion resistance, and which is not discolored duringbrazing. More particularly, the present invention provides a ferriticstainless steel that provides these benefits by adjusting theconstituent components of the ferritic stainless steel. As such, thepresent ferritic stainless steel can suitably replace a portion to whichan austenitic stainless steel is applied in a conventional EGR system.

According to one aspect, the present invention provides a ferriticstainless steel for an EGR system comprising iron (Fe) as a basematerial, about 18 to 20% by weight of chromium (Cr) based on a totalweight of an alloy (as used herein, “an alloy” refers to the componentsforming the ferritic stainless steel), and which can further include oneor more of molybdenum (Mo), carbon (C) and niobium (Nb).

According to various embodiments, a % by weight of the niobium (Nb) isabout 5 times or more % by weight as compared to the % by weight ofcarbon (C).

According to an exemplary embodiment of the present invention, theniobium (Nb) is present in an amount of about 0.2 to 0.7% by weightbased on the total weight of the alloy.

According to various embodiments, the ferritic stainless steel for anEGR system further comprises one or more of nitrogen (N) and carbon (C).According to an exemplary embodiment, the ferritic stainless steelfurther comprises more than 0 and up to about 0.01% by weight nitrogen(N), and more than 0 and up to about 0.02% by weight carbon (C) based onthe total weight of the alloy.

According to various embodiments, the ferritic stainless steel furtherincludes aluminum (Al). According to an exemplary embodiment, themolybdenum is present in an amount of about 0.75 to 1.5% by weight andaluminum (Al) is present in an amount of about 0.3 to 0.8% by weightbased on the total weight of the alloy.

According to further aspects, the present invention provides an EGRsystem comprising the ferritic stainless steel, particularly an EGRcooler, an EGR pipe, an EGR valve or an EGR bracket comprising theferritic stainless steel.

According the present invention, a ferritic stainless steel is providedthat reduces costs compared that of an austenitic stainless steel,particularly by including carbon (C), nitrogen (N), chromium (Cr),molybdenum (Mo), aluminum (Al) and niobium (Nb).

According to embodiments of the present invention, by excluding titaniumfrom the present composition, titanium nitride (Ti—N) is not produced onthe surface of the ferritic stainless steel during brazing, thusavoiding discoloration.

Further, according to embodiments of the present invention, moldabilityis secured by controlling the content of carbon (C), nitrogen (N) andniobium (Nb) in the ferritic stainless steel composition.

In addition, according to embodiments of the present invention,oxidation resistance and corrosion resistance are improved bycontrolling the content of molybdenum (Mo) and aluminum (Al) in theferritic stainless steel composition.

Other features and aspects of the present invention will be apparentfrom the following detailed description, drawings and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a photograph of an EGR cooler which is made of the ferriticstainless steel in the related art and, thus, is discolored.

FIG. 2 is a photograph of an EGR cooler which is manufactured of aferritic stainless steel for an EGR system according to an embodiment ofthe present invention and, thus, is not discolored.

FIG. 3 is an electron micrograph before the brazing of a ferriticstainless steel according to an embodiment of the present invention.

FIG. 4 illustrates the constituent components on the surface of aferritic stainless steel according to an embodiment of the presentinvention.

FIGS. 5 to 7 are photographs of a discolored site of the ferriticstainless steel in the related art, which are magnified 100 times, 200times, and 500 times, respectively through an electronic microscope.

FIG. 8 illustrates the constituent components of the discolored site ofthe ferritic stainless steel in the related art.

FIGS. 9 to 11 are photographs observed after brazing the ferriticstainless steel for an EGR system according to embodiments of thepresent invention, a conventional ferritic stainless steel and anaustenitic stainless steel, respectively.

FIGS. 12 to 14 are photographs of the corrosion resistance test resultsunder a heat treatment condition of 400° C., which is similar to anactuation environment of an EGR cooler of the ferritic stainless steelfor an EGR system according to embodiments of the present invention,after 10 days, 80 days and 160 days have passed.

FIGS. 15 to 17 are photographs of the corrosion resistance test resultsof the ferritic stainless steel in the related art under a heattreatment condition of 400° C. after 10 days, 80 days and 160 days havepassed.

FIGS. 18 to 20 are photographs of the corrosion resistance test resultof the austenitic stainless steel in the related art under a heattreatment condition of 400° C. after 10 days, 80 days and 160 days havepassed.

It should be understood that the appended drawings are not necessarilyto scale, presenting a somewhat simplified representation of variouspreferred features illustrative of the basic principles of theinvention. The specific design features of the present invention asdisclosed herein, including, for example, specific dimensions,orientations, locations, and shapes will be determined in part by theparticular intended application and use environment.

In the figures, reference numbers refer to the same or equivalent partsof the present invention throughout the several figures of the drawing.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Terms or words used in the present specification and claims should notbe interpreted as being limited to typical or dictionary meanings, butshould be interpreted as having meanings and concepts, which comply withthe technical spirit of the present invention, based on the principlethat an inventor can appropriately define the concept of the term todescribe his/her own invention in the best manner.

It is understood that the term “vehicle” or “vehicular” or other similarterm as used herein is inclusive of motor vehicles in general such aspassenger automobiles including sports utility vehicles (SUV), buses,trucks, various commercial vehicles, watercraft including a variety ofboats and ships, aircraft, and the like, and includes hybrid vehicles,electric vehicles, plug-in hybrid electric vehicles, hydrogen-poweredvehicles and other alternative fuel vehicles (e.g. fuels derived fromresources other than petroleum). As referred to herein, a hybrid vehicleis a vehicle that has two or more sources of power, for example bothgasoline-powered and electric-powered vehicles.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a,” “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof. As used herein, the term “and/or”includes any and all combinations of one or more of the associatedlisted items.

Unless specifically stated or obvious from context, as used herein, theterm “about” is understood as within a range of normal tolerance in theart, for example within 2 standard deviations of the mean. “About” canbe understood as within 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, 1%, 0.5%,0.1%, 0.05%, or 0.01% of the stated value. Unless otherwise clear fromthe context, all numerical values provided herein are modified by theterm “about”.

Hereinafter, the present invention will be described in detail based onTables and drawings.

FIG. 1 is a photograph of an EGR cooler which is manufactured of theferritic stainless steel in the related art, and FIG. 2 is a photographof an EGR cooler which is manufactured of the ferritic stainless steelfor an EGR system according to an embodiment of the present invention.As clearly shown, the EGR cooler manufactured of the conventionalferritic stainless steel is discolored, while the EGR coolermanufactured of the present invention ferritic stainless steel is notdiscolored.

According to a preferred embodiment of the present invention, theconstituent components of the ferritic stainless steel include iron (Fe)as a base material (in other words, iron is a main component and makesup the balance of the composition), carbon (C), nitrogen (N), chromium(Cr), molybdenum (Mo), aluminum (Al) and niobium (Nb). The constituentcomponents and contents of an embodiment of the present invention willbe investigated in detail through the Table 1.

TABLE 1 Physical properties Constituent components (% by weight based ontotal weight of the composition) YP TS EL Classification C N Cr Ni Ti MoAl Nb Fe (MPa) (MPa) (%) Hv Comparative 0.08 or — 18 to 20 8 to — — — —Remainder 205 450 59 or — Example 1 less 10.5 more Comparative 0.025 —16 to 19 — 0.30 0.75 to — — Remainder 300 470 32 or 155 Example 2 orless or less 1.5 more Present invention about about about — — aboutabout about Remainder 235 435 46 or — 0.02 or 0.01 or 18 to 20 0.75 to0.3 to 0.2 to more less less 1.5 0.8 0.7

Table 1 is a table that compares the constituent components, contents,and physical properties of Comparative Example 1, Comparative Example 2,and the present invention. YP means the yield point, TS means thetensile strength, EL means the elongation, and Hv means the hardness invickers.

Comparative Example 1 in Table 1 shows the constituent components,contents, and physical properties of the austenitic stainless steel(SUS304 and 300 series), and Comparative Example 2 shows the constituentcomponents, contents, and physical properties of the ferritic stainlesssteel (SUS436L and 400 series).

The present invention has been developed in order to solve thediscoloration problem during brazing of the ferritic stainless steel. Inparticular, according to the present invention, the constituentcomponents and contents of ferritic stainless steel, which is moreexpensive than the austenitic stainless steel, are controlled within aparticular range. As shown in Table 1, the ferritic stainless steel ofthe present invention does not include titanium (Ti), which isresponsible for the discoloration during brazing, which is in contrastwith the conventional ferritic stainless steel of Comparative Example 2.

FIG. 3 is an electron micrograph before the brazing of the conventionalferritic stainless steel, and FIG. 4 illustrates the constituentcomponents on the surface of the ferritic stainless steel of FIG. 3 witha graph and a Table. Iron (Fe) and chromium (Cr) are usually detected inthe components on the surface thereof because the ferritic stainlesssteel includes iron (Fe) as a base material and chromium (Cr) in thelargest amount.

FIGS. 5 to 7 are photographs of a discolored site of the conventionalferritic stainless steel of FIG. 3 after the brazing thereof, which aremagnified 100 times, 200 times, and 500 times, respectively, through anelectronic microscope. FIG. 8 illustrates the constituent components ofthe discolored sites of the ferritic stainless steel with a graph and aTable. It can be confirmed that in FIG. 8, which shows the constituentcomponents of the discolored site, that titanium (Ti) has beenconsiderably increased in comparison with FIG. 4 which shows theoriginal constituent components which are not discolored (i.e. prior tobrazing). Accordingly, it was demonstrated that titanium (Ti) is themain factor of the discoloration of the ferritic stainless steel.

Hereinafter, the principle through which titanium (Ti) changes the colorof the conventional ferritic stainless steel will be investigated. AnEGR system is manufactured through brazing, which is one of the methodsof bonding a metal in a batch furnace or continuous furnace. Titanium(Ti), which is one of the constituent components of the conventionalferritic stainless steel, reacts with nitrogen (N₂) during brazing toform dark titanium nitride (Ti—N) on the surface of the stainless steel.As a result, the ferritic stainless steel is discolored.

According to an aspect of the present invention, a ferritic stainlesssteel composition is provided which does not include titanium (Ti). Assuch, titanium nitride, which is responsible for discoloration, is notproduced during the brazing of the ferritic stainless steel.

However, the use of titanium in stainless steel provides a benefit inthat the titanium (Ti) bonds with carbon (C), thus stabilizing thecarbon (C) and strengthening the stainless steel. By elimination of thetitanium (Ti), titanium (Ti) and carbon (C) do not bind, the carbon (C)becomes unstable, and the strength of the stainless steel becomesweaker.

According to the present invention, in order to stabilize the carbon(C), niobium (Nb) is added. According to preferred embodiments, theamount of niobium (Nb) added is about 5 times or more % by weight theamount of carbon (C) added. Such amounts facilitate the stabilization ofthe alloy (i.e. the ferritic stainless steel composition).

Hereinafter, in the present invention, the role of each constituentcomponent and their preferred amounts will be described in furtherdetail.

The carbon (C) reacts with niobium (Nb) to form carbon nitride, whichincreases the strength of the stainless steel. However, when carbon (C)is added in a content of more than about 0.02% by weight based on thetotal weight of the ferritic stainless steel composition, moldabilityand corrosion resistance deteriorate and the precipitation of carbonnitride including niobium (Nb) results to reduce the high temperaturestrength. As such, the content of carbon (C) is preferably more than 0and up to about 0.02% by weight based on the total weight of theferritic stainless steel composition.

Further, the nitrogen (N) serves to increase the strength of thestainless steel by forming carbon nitride. However, but when thenitrogen (N) is added in an amount of more than about 0.01% by weightbased on the total weight of the ferritic stainless steel composition,the moldability and corrosion resistance deteriorate. As such, thecontent of nitrogen (N) is preferably more than 0 and up to about 0.01%by weight based on the total weight of the ferritic stainless steelcomposition.

In addition, the chromium (Cr) is an important element in improving thehigh temperature strength, oxidation resistance and corrosion resistanceof the stainless steel. According to preferred embodiments, the chromium(Cr) is included in an amount of about 18% by weight or more in order toobtain sufficient high temperature strength, oxidation resistance, andcorrosion resistance. However, when the content of chromium (Cr) exceedsabout 20% by weight, the strength of the stainless steel is increased toreduce the elongation, thereby leading to a deterioration ofmoldability. As such, the content of chromium (Cr) is preferably about18 to 20% by weight.

Furthermore, the molybdenum (Mo) is an important element in improvingthe high temperature strength and corrosion resistance of the stainlesssteel. According to preferred embodiments, the molybdenum (Mo) isincluded in an amount of about 0.75% by weight or more in order toobtain sufficient oxidation resistance, corrosion resistance, and hightemperature strength. However, when the content of molybdenum (Mo)exceeds about 1.5% by weight, the tenacity thereof deteriorates. Assuch, the content of molybdenum (Mo) is preferably about 0.75 to 1.5% byweight.

Further, the aluminum (Al) is an element which is added as a deoxidizerof the stainless steel. According to preferred embodiments, the aluminum(Al) is included in a content of about 0.3% by weight or more in orderto improve the oxidation resistance, corrosion resistance, and hightemperature strength of the stainless steel. However, when the contentthereof exceeds about 0.8% by weight, the stainless steel is hardened,thus resulting in cracks occur and reduction in tenacity. Accordingly,the content of aluminum (Al) is preferably about 0.3 to 0.8% by weight.

In addition, the niobium (Nb) serves to stabilize the stainless steel byfixing carbon (C) and nitrogen (N) as a carbon nitride, and is also anelement effective in improving the high temperature strength. Accordingto preferred embodiments, the niobium (Nb) is included in a content ofabout 0.2% by weight or more in order to obtain sufficient hightemperature strength. However, when the content exceeds about 0.7% byweight, the tenacity deteriorates. As such, the content of niobium (Nb)is preferably about 0.2 to 0.7% by weight.

The ferritic stainless steel for an EGR system according to the presentinvention may be appropriately prepared by those skilled in the artusing any known technology, and may be extensively applied as a materialthat requires corrosion resistance for an EGR cooler, an EGR pipe, anEGR valve, an EGR bracket and the like.

EXAMPLE 1

Hereinafter, the present invention will be described in more detailthrough the Examples. These Examples are only for illustrating thepresent invention, and it will be obvious to those skilled in the artthat the scope of the present invention is not interpreted to be limitedby these Examples.

FIGS. 9 to 11 are photographs taken after brazing the ferritic stainlesssteel for an EGR system according to an embodiment of the presentinvention, a conventional ferritic stainless steel, and a conventionalaustenitic stainless steel, respectively. As demonstrated in FIGS. 9 to11, the ferritic stainless steel of the present invention and theaustenitic stainless steel, both of which do not include titanium (Ti),are not discolored during the brazing thereof. This is in contrast withthe conventional ferritic stainless steel in which titanium (Ti) isincluded and which shows discoloration.

Furthermore, FIGS. 12 to 14 are photographs of the corrosion resistancetest results under a heat treatment condition of 400° C., which issimilar to an actuation environment of an EGR cooler, of the ferriticstainless steel for an EGR system according to the present invention,after 10 days, 80 days and 160 days have passed.

FIGS. 15 to 17 are photographs of the corrosion resistance test resultsof the conventional ferritic stainless steel under a heat treatmentcondition of 400° C. after 10 days, 80 days and 160 days have passed.

FIGS. 18 to 20 are photographs of the corrosion resistance test resultsof the conventional austenitic stainless steel under a heat treatmentcondition of 400° C. after 10 days, 80 days and 160 days have passed.

As demonstrated by the tests, the ferritic stainless steel for an EGRsystem according to the present invention has corrosion resistance atleast equivalent to that of the conventional austenitic and ferriticstainless steels. On particular, corrosion does not proceed in theferritic stainless steel for an EGR system according to the presentinvention.

Hereinafter, the test results of FIGS. 12 to 20 will be investigated inmore detail through Tables 2 and 3.

TABLE 2 10 days 80 days Residual Residual 160 days thickness thicknessResidual thickness Classification (%) (%) (%) Comparative Example 1 100100 100 Comparative Example 2 100 100 100 Example 1 100 100 100

TABLE 3 10 days 80 days 160 days Residual Residual Residual weightClassification weight (%) weight (%) (%) Comparative Example 1 100 100100 Comparative Example 2 100 100 100 Example 1 100 100 100

Tables 2 and 3 are tables that compare the residual thickness (%) andthe residual weight (%) after an experiment is performed under a heattreatment condition of 400° C., which is similar to an actuationenvironment of an EGR cooler in order, to investigate the corrosionresistance of Comparative Example 1, Comparative Example 2, andExample 1. Comparative Example 1 means the conventional austeniticstainless steel , and Comparative Example 2 refers to the conventionalferritic stainless steel. Example 1 is the ferritic stainless steelaccording to an embodiment of the present invention, based on theconstituent components and contents of the present invention of Table 1.

Each of the residual thickness (%) and residual weight (%) ofComparative Example 1, Comparative Example 2, and Example 1 was measuredafter 10 days, 80 days and 160 days of the start of the test had passed,and as a result of the test, it was shown that Example 1 securedcorrosion resistance at least equivalent to those of Comparative Example1 and Comparative Example 2 from the fact that Comparative Example 1,Comparative Example 2, and Example 1 all have a residual thickness and aresidual weight of 100%, respectively.

As described above, the present invention has been described in relationto specific embodiments of the present invention, but this is onlyillustration and the present invention is not limited thereto.Embodiments described may be changed or modified by those skilled in theart to which the present invention pertains without departing from thescope of the present invention, and various alterations andmodifications are possible within the technical spirit of the presentinvention and the equivalent scope of the claims which will be describedbelow.

What is claimed is:
 1. A ferritic stainless steel for an EGR system,comprising: iron (Fe) as a base material, 18 to 20% by weight ofchromium (Cr) based on a total weight of an alloy forming the ferriticstainless steel, molybdenum (Mo), carbon (C) and niobium (Nb).
 2. Theferritic stainless steel for an EGR system of claim 1, wherein a % byweight of the niobium (Nb) is about 5 times the % by weight of carbon(C) based on the total weight of the alloy.
 3. The ferritic stainlesssteel for an EGR system of claim 1, comprising about 0.2 to 0.7% byweight niobium (Nb) based on the total weight of the alloy.
 4. Theferritic stainless steel for an EGR system of claim 1, furthercomprising: nitrogen (N), wherein the carbon (C) is present in an amountof more than 0 and up to about 0.02% by weight, and the nitrogen (N) ispresent in an amount of more than 0 and up to about 0.01% by weightbased on the total weight of the alloy.
 5. The ferritic stainless steelfor an EGR system of claim 1, further comprising: aluminum (Al), whereinthe molybdenum (Mo) is present in an amount of about 0.75 to 1.5% byweight, and aluminum (Al) is present in an amount of about 0.3 to 0.8%by weight.
 6. The ferritic stainless steel for an EGR system of claim 1,which does not include titanium (Ti).
 7. The ferritic stainless steelfor an EGR system of claim 1, wherein the ferritic stainless steel isapplied to an EGR cooler, an EGR pipe, an EGR valve or an EGR bracket.